The present invention relates to a beam shutter designed for industrial excimer lasers including a rotating mirror for selectively opening and closing the beam path.
High power lasers require laser shutters which can withstand up to 300 W output power. A prior art design consists of a large metal piece which can be pulled upwards out of the beam path by an electrically or pneumatically activated actuator. De-energizing the actuator results in closing the beam path by sliding down the metal piece. A common choice for the metal beamstop is an extruded aluminum heat sink guided by sliding rails or rod-roller bearings. For high power XeCL lasers it is advantageous to either plate the metal beam stop with copper or to use a solid copper beam stop. Copper has a lower ablation rate than aluminum and therefore contamination of laser and beam line optics is reduced. However, there are several disadvantages of this high power shutter design. First, the heavy metal beam stop results in slow opening and closing times. Further, the large heat load on the moving metal beam stop causes significant thermal expansion of the beam stop so that the sliding rails or rod-roller bearings of the beam stop must have large clearance to permit the up and down movement. Such large clearances are contradictory to precise shutter movement. In addition, the shutter is not light tight to the required extent because of the large required clearance between moving parts. Further, the removal of the large heat load requires a large opening for ventilation, which yields a design which cannot be integrated into a purged beam path with low losses of purge gas.
One state of the art beam shutter (U.S Pat. No. 5,596,590) has a linear slide base guided by a pair of rod-roller bearing supports. Making this shutter light tight requires precise alignment of the moveable mirror base relative to the shutter housing. Additionally, the pair of rod-roller bearing supports must be precisely aligned. The required precision is in the order of 0.1 mm. Low fit tolerances of all mechanical parts are mandatory. The precision alignment of the moveable mirror base and low fit tolerances are disadvantages to this type of shutter design. Therefore, it is desirable to have a shutter that achieves precision alignment for light tightness without cumbersome alignment procedures.
Another state of the art shutter has two metal blades that interrupt the beam path. This type of shutter is very fast, with opening and closing times of less than 100 ms. However, when the metal blades interrupt the laser, the laser power is partially reflected and partially absorbed by the metal blades. Consequently this type of design can handle only very low average power levels. Dielectrically coated metal blades increase the power handling capability of this shutter type significantly. However, in the ultraviolet spectral range, the coatings are not perfect and a portion of UV laser beam will still reach the metal blades. Therefore, the shutter can not handle the required level of UV light. Consequently it is desirable to have a beam shutter that can handle high levels of UV light.
Also, there is a state of the art shutter that consists of a hollow metal cylinder preferably made of a metal with low specific gravity rotated by an electrically powered actuator. The hollow cylinder has two rectangular openings symmetrically positioned to the cylinder axis. Depending on the rotational position of the cylinder, the laser beam could either pass through the two rectangular openings or be blocked by the solid section of the cylinder. The thin wall metal cylinder has a low moment of inertia which results in a fast opening and closing time of about 100 ms. However the disadvantage of this shutter is the low power handling capability and the high level of stray light caused by the beam reflected off the solid section of the hollow cylinder. It is more desirable to have a shutter that has a high power handling capability and is light tight.
An object of the present invention is to provide a beam shutter which can handle high powered UV radiation.
It is a further object of the present invention to provide a beam shutter that does not require precise alignment of moveable parts.
It is a another object of the present invention to provide a beam shutter that is leak tight regarding purge gas, making purge gas loss extremely low.
The objects and features of the present invention are realized by the use of a beam shutter having a high reflectivity mirror mounted on the top side of a plate which is rotated into the beam path in order to interrupt the beam and reflect it to a side port, to which a beam dump or diagnostic tool is attached. The beam dump or diagnostic tools are attached to the side port by rotation using a bayonet-type fitting.
The shutter has a base including an angled upper surface for supporting the plate and mirror at an angle for reflecting the laser beam to the side port. The shutter base has a beam channel through it to allow a beam to pass unimpeded when the plate and mirror are rotated to an open position away from the base. The plate carrying the mirror is equipped with a labyrinth seal on its underside to achieve light tightness when positioned on the shutter base.
Because the upper surface of the shutter base is machined at a predetermined angle, there is no need for precise motor control for positioning the mirror for reflection. The mirrored plate is simply rotated into contact with the upper surface of the shutter base. The labyrinth seal under the plate fills and seals the beam channel in the shutter base for light tightness. When no reflection is desired, the mirror is rotated out away from the channel an amount sufficient to allow the beam to pass.
The plate is connected to a linkage, which rotates the plate and mirror into and out of position for reflecting the beam. The linkage is comprised of several members connected by lubricant free bearings, and is operated by a rotational magnet, which is attached to the shutter housing. The design of the shutter base and linkage eliminates the requirement for the precision alignment of mechanical parts. Also the shutter body including the bearing seats are machined out of a monolithic piece of metal, which further allows precision without other cumbersome alignment features.